Intensity Of Reinfection Research Articles

Age-dependency in predisposition to hookworm infection in the Burma valley area of Zimbabwe

We examined hookworm re-infection intensities (measured as faecal egg count by the Kato technique) following anthelmintic treatment of farm workers resident on 15 large-scale commercial farms in Zimbabwe. All individuals examined were treated with albendazole (400 mg). Hookworm reinfection intensities of individuals examined, treated and reexamined were compared with the intensities of infection before treatment. Adults had higher than average infection intensities both before and after reinfection. Predisposition to hookworm infection was marked in adults, and absent from younger, less heavily infected hosts. The observations suggest that acquired immunity is unlikely to be an important determinant of hookworm infection and that behaviour may play a major role in determining the observed infection patterns in this community.

Longitudinal studies on human schistosomiasis.

A major difficulty in understanding the epidemiology of human schistosomiasis has been to distinguish between acquired immunity and reduced exposure as possible reasons for an observed decline, in older individuals, of levels of superinfection or of reinfection after chemotherapy. A series of studies of Schistosoma mansoni infections in Kenya has been undertaken to approach this problem, by investigation of intensities of reinfection after treatment of individuals whose levels of contact with contaminated water is subsequently observed. Intensities of reinfection are highest among younger children, thereafter declining sharply. This decline can be attributed only in part to age-related changes in the duration and nature of exposure; there is also evidence for the development of an acquired resistance to reinfection that is dependent both on age and on previous experience of infection, and that may be immunologically mediated. Evidence has been obtained that the slow development of this acquired immunity with age may be associated with the early development and subsequent slow decline of inappropriate immune responses that 'block' the effect of potentially protective responses. Implications of these findings for immunological intervention through vaccination are discussed.

Rate of re-infection by Opisthorchis viverrini in an endemic Northeast Thai community after chemotherapy

Patterns of re-infection by the human liver fluke, Opisthorchis viverrini, were studied over a period of 1 year after chemotherapy in a community in Northeast Thailand. Praziquantel (40 mg per kg body weight) was administered to 808 subjects; 88.4% of those examined after 2 weeks had negative stools. Within 1 year of treatment, however, 87.7% of the cured subjects who were re-examined had become re-infected, and 51.5% had infections of at least moderate intensity (1 epmg of faeces), in comparison with 72% of the same group before treatment. Those with high pre-treatment intensities of infection tended to have heavier intensities of re-infection, indicating that some people are predisposed to heavy infections. The rate of re-infection of those with heavy pre-treatment intensity of infection was about twice that of those who were negative or had only light infection before treatment. The rate of re-infection was markedly higher than concurrently and previously measured natural incidences of infection. These findings suggest that chemotherapy would have to be applied several times a year in order to control opisthorchiasis, and that it might be most cost-effective to preferentially treat heavily infected individuals.

Predisposition of humans to infection with Schistosoma mansoni: evidence from the reinfection of individuals following chemotherapy

In a study of faecal egg counts of Schistosoma mansoni from 359 people of all ages from a rural Kenyan community, a positive association was demonstrated between infection intensity in individuals before treatment and reinfection intensity in the same individuals 9 months after treatment in certain age groups of the sampled population. Consequences and possible causes of these observations are discussed in terms of the epidemiology and control of schistosomiasis.

Immunity in human schistosomiasis mansoni: prevention by blocking antibodies of the expression of immunity in young children

A total of 129 children were treated for Schistosoma mansoni infections, and followed for intensity of reinfection at 3-monthly intervals over a 21-month period. Blood samples were taken before treatment and at 5 weeks and 6, 12 and 18 months after treatment. This paper presents a statistical analysis of the relationship between various immune responses and subsequent reinfection. Responses analysed were: blood eosinophil levels; IgE antibodies against schistosomulum antigens; IgG antibodies mediating eosinophil-dependent killing of schistosomula; antibodies inhibiting the binding to schistosomulum antigens of two rat monoclonal antibodies that also recognize egg antigens; the levels of anti-adult worm and of anti-egg (total, IgM and IgG) antibodies; and IgM anti-schistosomulum antibodies. Results for each assay were well correlated for each of the five separate blood samples. None of the assays were predictive of resistance to reinfection, but susceptibility to reinfection was strongly correlated with results in the preceding blood samples for total anti-egg antibodies and the inhibition of binding of one of the two monoclonal antibodies. Further analysis also revealed a correlation between reinfection intensities and both IgM anti-schistosomulum antibodies and IgM and IgG anti-egg antibodies. These results are consistent with the hypothesis that early infections elicit the development, in response to egg antigens, of antibodies that block immune mechanisms directed against schistosomula. Blocking antibodies may be IgM, but might also be of an ineffective IgG isotype. The existence of such antibodies in young children would explain the slow development of immunity in the face of a range of detectable, potentially protective immune responses.

Immunity after treatment of human schistosomiasis mansoni. II. Identification of resistant individuals, and analysis of their immune responses

Intensities of re-infection were monitored at three-monthly intervals after treatment of Schistosoma mansoni infections in a group of 119 Kenyan schoolchildren, whose levels of water contact were also observed. 22 children showed high reinfection intensities (>100 eggs per gram of faeces) by 12 months after treatment, and were considered to be susceptible. Out of 70 children who showed low reinfection intensities during the same period (<30 eggs per gram), 35 showed high levels both of total water contact and of contact with sites containing infected snails. In these children, the relative lack of reinfection could not be attributed to a lack of exposure, and they were classified as resistant to reinfection. Comparison of the two groups, resistant and susceptible, revealed no difference in pretreatment intensities of infection. However, there was a marked difference in age, the mean age of the resistant group being two years greater than that of the susceptible group, within a restricted starting age range. These findings indicated that resistance was an acquired and age-dependent phenomenon, not obviously related to previous egg-induced pathology. Studies of immune responses revealed no clearcut correlate of resistance, but there were interesting differences between the two groups. Whereas anti-egg antigen responses declined after treatment to a greater extent in the resistant than in the susceptible group, antibodies mediating eosinophil-dependent killing of schistosomula rose markedly in both groups, strongly suggesting that the resistant children were being exposed to cercariae. Anti-adult worm antibodies rose sharply in both groups immediately after treatment, and thereafter declined to pretreatment levels. Although some individual children showed high levels of IgE anti-schistosomulum antibodies, there were no significant differences between the two groups. Since all children showed detectable levels of antibodies mediating eosinophil-dependent killing of schistosomula, the possibility was considered that such antibodies might be a necessary, but not a limiting, factor in immunity. Instead, the functional state of the effector cells mediating antibody-dependent killing might be limiting. Eosinophil levels, measured as an indirect estimate of eosinophil functional activity, did not differ between the two groups. There were, however, marked differences between different individuals in their capacity to produce eosinophil-stimulating monocyte mediators, and although this cannot yet be related to resistance, this aspect is worth further study. The ability of resistant and susceptible individuals to recognize different antigens was also studied, by immunoprecipitation of adult worm mRNA in vitro translation products. There was a marked heterogeneity in the responses of different individuals to different polypeptides, some being recognized by most sera, others by only a few. A somewhat higher proportion of resistant than of susceptible children recognized two polypeptides, of 27 and 24·5 Kd respectively, that might be useful candidates for testing as “protective” antigens following cloning and expression in Escherichia coli.